Agriculture Reference
In-Depth Information
Bacteria also contribute to litter decomposition by releasing a large variety of enzymes in the
environment.
The knowledge of the interaction between fungi and bacteria is important to understand
the decomposition process. Nevertheless, generally, the fungal biomass exceeds the bacterial
component mainly in the initial stage of cellulose, lignin and chitin decomposition (Swift et
al., 1979).
In recent years, a greater attention has been turned to microbial degrading capacity by
considering the extracellular enzymatic activities (Fioretto et al., 2000; Sinsabaugh et al.,
2002; Kjøller and Struwe 2002; Di Nardo et al., 2004). The study of the decomposition
dynamics in relation to the enzymes that degrade the major structural constituents of plant
material, may provide information on specific metabolic and functional aspects of microbial
communities (Sinsabaugh et al., 1991).
Microbial communities release hundred of different enzymes into the environment. Those
directly involved in the degradation of lignocellulose (cellulases and phenoloxidases) and in
N, P and S cycles are of primary interest. The degradation of the major structural constituents
of plant organic matter requires multi-component enzyme systems, involving different
microbial taxa. Cellulose and lignin degradation, for example, requires glucose oxidase and
cellobiose dehydrognase that oxidize glucose and cellobiose, respectively. Glucose oxidase
generates H
2
O
2
, which is required by ligninolytic peroxidases, while cellobiose
dehydrogenase reduces quinones generated by the oxidative depolymerization of lignin.
Glucose and cellobiose are produced by cellulose hydrolysis and, both, are feedback
inhibiters of cellulolysis (Sinsabaugh et al., 1991). The substrates also interact as well as the
enzymes. Cellulose, lignin and hemicellulose are covalently linked and physically intercalated
in plant cell wall (Marsden and Gray, 1986). So, extensive degradation of litter needs of the
concerted action of a variety of enzymes.
Laccases appear to be the best lignin degrading agents (Reid, 1995) and are released by
many fungi (Tuor et al., 1995), especially white-rot fungi. These enzymes act in synergy with
other polyphenoloxidases such as Fe-peroxidases or Mn-peroxidases. Cellulases are enzymes
that degrade cellulose. They are divided at least in 2 enzyme classes: endoglucanases
(endocellulase), which exhibit different affinities to cristalline cellulose and
cellobiohydrolases (exocellulase). In principle, the level of activity of cellulases should be
related to the rate of cellulose degradation in litter. However, the relationship is complex and
only an index of cellulase interactions (the product of exocellulase and endocellulase
activities) correlated with the rate of disappearance of cellulose from decomposing leaf litter,
both within and between species (Linkins et al., 1990; Sinsabaugh et al., 1994).
Chitinases catalyze hydrolysis of chitin that is the main structural component of cell walls
of most fungi and arthropods. They are produced by many species of bacteria (streptomycetes
and other actinomycetes), fungi as well as plants and play an important physiological and
ecological role in ecosystems as recyclers of chitin, by releasing C and N nutrients (Chernin
and Chet, 2002).
By considering the large number of factors involved and interacting, litter decomposition
is a complex process that requires an interdisciplinary approach.
